Organic Light-Emitting Diodes (OLEDs) are honored as a new generation of flat panel displays. In comparison to current mainstream Liquid Crystal Displays (LCDs), the organic light-emitting diodes have the advantages of self illumination, wide angle of view, high contrast ratio, short response time, thin panel thickness (flattening), flexible display and the like. The basic structure of an organic light-emitting diode comprises an anode, a cathode and a light-emitting layer between the anode layer and the cathode layer. Under the action of an external voltage, electrons and holes are injected from the cathode and the anode respectively, then are transferred and encounter with each other in the light-emitting layer to generate excitons. The energy of the excitons is attenuated in the form of light, namely, the excitons radiate light.
Since METHOD FOR MANUFACTURING MULTI-LAYER THIN FILM OLED BY VACUUM EVAPORATION was published by Doctor Qingyun Deng in 1987, the OLED has been attracted extensive attention and widely researched in both educational circles and industrial circles. However, many problems remain to be improved at present, in which how to improve the light extraction efficiency of the OLED is still one of key points. At present, to improve the light extraction efficiency, there are generally two methods: one method is to improve internal quantum efficiency, while the other method is to improve external quantum efficiency. When photons are incident on the surface of photosensitive equipment, a part of the photons will be absorbed and thus excite the photosensitive material to generate electron hole pairs so as to form current. In this case, a ratio of the number of the generated electrons to the number of the absorbed photons is the internal quantum efficiency, and a ratio of the number of the generated electrons to the number of all the incident photons is the external quantum efficiency.
The internal quantum efficiency mainly measures a ratio of the number of a part of excitons, which are generated during recombination of injected current carriers in a light-emitting layer and turned into photons for coupling luminescence to the total number of the excitons. The improvement of the internal quantum efficiency may be achieved by improving material performance, using phosphor materials or other ways. Theoretically, the luminescence with the internal quantum efficiency approximate to 100% may be reached.
To improve the external quantum efficiency is to improve the coupling light extraction efficiency of an OLED. For a flat panel display device, only about 20% of photons can be generally extracted out for the exciton luminescence, but the vast majority (80%) of energy is lost in various modes, such as a substrate mode occurring at refracting or reflecting boundaries between the anode of the OLED and a substrate or between the substrate and air, a waveguide mode occurring between the anode of the OLED and a light-emitting layer boundary, and a Surface Plasmon (SP) mode occurring near a metal electrode, wherein more than 40% of light is limited in an OLED due to the SP mode, light limited by the waveguide mode and light limited by the substrate mode are 15% and 23%, respectively. As the loss caused by metal absorption is 4%, only about 20% of light emitted from a light-emitting layer may transmit out from the OLED, then enters air and is thus seen by human eyes.
At present, the loss caused by the waveguide mode is reduced by additionally arranging a micro lens or a micro-cavity structure on the surface of a substrate, or the loss caused by the substrate mode is improved by additionally arranging an optical grating or photonic crystal on a substrate to reduce total reflection, or the light extraction efficiency is improved by Bragg diffraction or other technologies. However, the micro lens generally only can realize the improvement of the light extraction efficiency in the illumination field, but is still not beneficial in the display field at the level of pixels having fine size. The use of the micro-cavity structure generally will result in the deviation of the outgoing light color of the OLED and a narrower angle of view, and the photonic crystal requires a complicated photo-etching process, is difficult to be prepared and realized, and meanwhile may further cause color offset and other problems of view angle due to the presence of optical gratings and the like. The Bragg diffraction technology generally requires that emergent light is adjusted by alternately laminating multiple layers of materials having high and low refractivity with thicknesses of high precision; meanwhile, different light-emitting colors (such as red R, green G and blue B) require different best thicknesses of Bragg diffraction layers, so the precise adjustment of the thicknesses of RGB must be realized by multiple steps of depositions, mask exposures and etching processes, which has large difficulty, low yield and high cost for the preparation technology of full-color OLED display devices.
It can be seen from the above that how to couple out light lost by the SP mode having the largest loss ratio is a method for effectively improving the external quantum efficiency of an OLED device; meanwhile, for a full-color OLED display device, how to use a simple and practical method to improve the light extraction efficiency of red, green and blue sub-pixels at the same time is also a technical problem urgent to be solved in the OLED field at present.